1. Field of the Invention
The present invention relates to a channel estimation method, more particularly to a multi-symbol channel estimation method for estimating channel response in a multi-carrier modulation system.
2. Description of the Related Art
Generally, a multi-carrier modulation (MCM) system, such as an orthogonal frequency division multiplexing (OFDM) system, is used for broadband data transmission. In the OFDM system, interference attributed to multi-path channels is effectively resolved through transmission of orthogonal long symbols. However, orthogonality of the symbols will fail when the OFDM system is used in fast time-varying selective fading channels, especially under a high-mobility environment. Since enhancement of Doppler effect will increase a time-varying rate of a time-varying channel, and the faster time-varying rate will result in the more obvious failure of the orthogonality among subcarriers within each of the symbols, a channel matrix representing the time-varying channel will no longer be a diagonal matrix. Thus, use of a conventional one-tap frequency-domain equalizer for estimating channel response usually results in relatively worse performance of the system.
Currently, there are many techniques used for estimating channel response in the MCM system, and various methods of insertion of pilots are used for facilitating the estimation of the channel response. For example, S. Chen et al. proposed a method of channel estimation (referred to as Chen method hereafter) in “Intercarrier interference suppression and channel estimation for OFDM systems in time-varying frequency-selective fading channels,” IEEE Trans. Consumer electron, Vol. 50, No. 2, pages 429-435; Y. Mostofi et al. proposed a method of channel estimation (referred to as Yasamin method hereafter) in “ICI mitigation for pilot-aided OFDM mobile systems,” IEEE Trans. Wireless Commun., Vol. 4, No. 2, pages 765-774; Huang-Chang Lee et al. proposed a method of symmetric training-symbol-based channel estimation in “Channel estimation for OFDM system with two training symbols aided and polynomial fitting,” IEEE Transactions on Commun., Vol. 58, No. 3, pages 733-736; and Chao-Wei Chen et al. proposed a method of asymmetric training-symbol-based channel estimation in “Channel estimation for OFDM systems with asymmetric pilot symbols,” IEEE Wireless Communications and Networking Conference, April 2010, pages 1-5.
Chen method and Yasamin method involve insertion of the pilots into certain subcarriers in a frequency domain. Chen method utilizes a single symbol to serve as an observation window to estimate the channel response. Namely, Chen method is a single-symbol-based channel estimation method, and can only be used for estimating the channel response to the single symbol during transmission using a set of first-order polynomial interpolations. Therefore, there is a requirement of a relatively greater number of pilots within the symbol, that is to say, high overhead is required for implementing Chen method since Chen method utilizes the first-order polynomial interpolations. On the other hand, Yasamin method involves messages interpolation in cyclic prefixes of adjacent symbols, or uses pilots at symbol midpoint of adjacent symbols for estimating channel response using a first-order polynomial. As a result, a number of the pilots is relatively smaller, and is limited to a certain number corresponding to a number of the subcarriers. For the above reasons, the channel response estimated using Chen method or Yasamin method under a high-mobility environment is relatively inaccurate since these two methods merely use the first-order polynomial during estimation of the channel response to a fast time-varying channel.
The method of symmetric training-symbol-based channel estimation involves insertion of a pair of training symbols (i.e., full-overhead pilots) respectively before and after a set of symbols in an observation window in a-frequency domain. Then, the set of symbols between the two training symbols in the observation window are data symbols. The method of asymmetric training-symbol-based channel estimation is similar to the method of symmetric training-symbol-based channel estimation. In the method of asymmetric training-symbol-based channel estimation, one of the training symbols is a partial-overhead pilot. Although these two methods are capable of enhancing the accuracy of the estimated channel response, at the same time, pilot overhead is significantly increased and computation is complicated. As a result, transmission efficiency is reduced.